pktcdvd: Remove CONFIG_CDROM_PKTCDVD_WCACHE from uapi header

CONFIG_* switches should not be exposed in uapi headers, thus let's get
rid of the USE_WCACHING macro here (which was also named way to generic)
and integrate the logic directly in the only function that needs it.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>